Fix possible deadlock in unix/socket.c

Fix possible deadlock in unix/socket.c

In process_fd we lock sock->lock and then internal_accept locks mgr->lock,
in isc_sockmgr_render* functions we lock mgr->lock and then lock sock->lock,
that can cause a deadlock when accessing stats. Unlock sock->lock early in
all the internal_{send,recv,connect,accept} functions instead of late
in process_fd.